OBJECT-GLASSES. xvii 



with, the distinctness of the image of the object. The various parts of an object lying in 

 the same plane, as a transverse section of whalebone, should also be visible at the same 

 focus ; the lines upon a micrometer used as a slide will also serve to test this point. It is not, 

 however, of very great importance, especially with high powers ; but it is a character of a 

 superior object-glass. 



If the definition of the glass be good, the field flat, and the power adequately high, it 

 will also exhibit the structure of the objects in Plate 1. figs. 5, 6, 10, 12, and 13 clearly and 

 distinctly ; it is then of sufficiently good quality for nearly all the purposes required in the 

 investigation of animal and vegetable structures. 



The exhibition of the objects illustrated by Plate 1. figs. G, 7, 8, 9, 10, 11, 12, and 13 

 requires the first kind of penetrating power, but it does not require large angular aperture. 

 The second kind of penetration, however, requires, above all, large angular aperture, inde- 

 pendently of any other superiority; i. e. a glass may be perfectly corrected as to defining 

 power, and exhibit the above objects well, yet when the valve of a Pleurosiyma is sub- 

 jected to it the markings cannot be distinguished without particular appliances, which 

 produce the same effect as an increase of angular aperture in the object-glass. As this 

 property is therefore principally dependent upon r the angular aperture, this should be de- 

 termined by direct measurement ; the method of doing which is described under the article 

 "ANGULAR APERTURE," in which also is contained a list of the various apertures of the 

 best glasses, so that the approximation in the case of any glass to these magnitudes will 

 afford an indication of its quality. It must be observed that increase of angular aperture 

 in an object-glass involves an increase in price. 



The following remarks may perhaps assist in guiding the judgment in regard to the 

 selection of an object-glass : 



Large angular aperture is of less importance in the case of a low than of a high power. 



Large angular aperture is neither requisite nor advantageous in physiological and medical 

 investigations in general. 



Whether a glass of larger aperture will exhibit any further structure than one of less 

 aperture has already done, can nearly always be predicted from other means. 



Object-glasses of high power and large angular aperture require to be brought very 

 close to the objects viewed, which is a great disadvantage, rendering them useless for 

 general investigations. 



In regard to objects requiring large angular aperture for exhibiting their structure, much 

 depends upon the management of the light : so that a glass may fail in exhibiting certain 

 parts of structure in the hands of one of but little experience, whilst in the hands of another 

 it may show them distinctly. Hence the direct measurement of the angle is best, to deter- 

 mine what a glass is capable of exhibiting when properly used. 



The student may perhaps find himself perplexed by the conflicting statements made by 

 different renowned observers in respect to object-glasses. The illustrious Schleiden said 

 that only a magnifying power of about 500 diameters is useful for scientific purposes, that 

 witli our present microscopes we may see whatever we like with a power of 3000, and that 

 only the amplification of an object to the extent of 280 or 300 diameters is produced by the 

 object-glass, all beyond this being effected by the eyepieces with an almost total ^oss of 

 light. These statements were perhaps formerly true ; but they do not apply to the modern 

 object-glasses. The highest modern object-glasses will show minute objects with a power 

 of from GOO to 2500 diameters with the lowest eyepiece, as clearly and well defined as the 



